This invention relates to electrical machines and generators, and more particularly to thermal management of high power density electrical machines.
In the field of power generation, high speed motors and power generators have been developed for various power generation applications. High speed motors and generators are advantageous to many challenging applications because their high-speed nature allows for greater power density over other types of power generators, among other advantages. High power density means that a comparable amount of power (over other types of power generators) is available in a much smaller volume and weight. Stated differently, with a high speed motor or power generation device, the overall size and weight of the device is reduced relative to other machines capable of providing the same amount of power/energy. This is advantageous in those applications where weight and size need to be minimized, for example, to make the device easily transportable.
However, there are also disadvantages of having an increased power density. One of the more important disadvantages is the generation of large amounts of heat per unit volume. High power density results in a high current density, which directly relates to Joule heating. Because of the smaller volume of the machine, this heat can become problematic, as getting the heat out of the machine is difficult. Moreover, if the heat is not removed effectively, the machine will eventually fail.
An additional disadvantage with high-speed, high power density machines is the losses created by the high frequency of the alternating current. The use of high frequency alternating current means that smaller cross-section conductors (made of copper filaments for example) must be used to carry the current. The use of small strand diameters is used to minimize these losses. However, the use of these smaller strand/conductor diameters makes it more difficult to remove the heat from high power density machines.
As stated above, the heat load must be removed efficiently to protect the electrical system and primarily the electrical insulation system. Namely, for the machine to operate, electrical circuits must be maintained, and electrical insulation between strands, between turns, and between coils are necessary for this task. If the temperature limits of the insulation are exceeded, the insulation will break down and a short can occur. This can lead to a failure of the entire machine.
Previous methods to remove heat from power generation devices include using hollow strands or conductors and passing a heat transfer fluid through the conductors. However, as indicated above in high-speed, high power density machines there is a need to reduce strand size, and thus this methodology can not be effectively used.
Other methods include natural or forced convention (i.e. air cooling). However, in high power density machines both of these methods are insufficient for removing the large amount of heat generated.
An additional method uses indirect liquid cooling. In this method heat is conducted through various paths to reach the liquid coolant, but the liquid coolant does not directly contact the strands/conductors. In this method, liquid is pumped through a metal tube, which is placed near the conductors. However, because the tube is metal it must be insulated (to prevent electrical shorting), and this insulation causes the temperature to rise in the conductors, as the insulation essentially acts as a thermal barrier. Moreover, the wall thickness of the metal tubes often needs to be relatively thick to ensure proper function, thus adding to the relative size and weight of the power generator.
Therefore, there still exists a need to effectively and efficiently remove a high amount of heat from high power density machines.